R. K. Wild

Characterization of iron/oxygen surface reactions by X-ray photoelectron spectroscopy

Abstract The initial stages of oxidation of clean, high-purity, iron foil have been studied using a Kratos ES 300 electron spectrometer with monochromatic X-radiation. Through careful observation of the Fe 2p peak profile, peak energy maximum, and peak satellite structure, it was possible to identify oxide species containing Fe (II) and Fe (III). The results indicated that iron was highly mobile in the Fe (II)/Fe (III) lattice at intermediate oxidation levels and that the iron ions in the octahedral sites of Fe3O4 exhibited a rapid symmetric intervalence electron transfer of the type Fe (II) + Fe (III) → Fe (III) + Fe (II).

High resolution lmm auger electron spectra of some first row transition elements

Abstract Auger spectra have been recorded from elements of the first transition series using a hemispherical analyser. Highly resolved LMM spectra were obtained showing for the first time the composite nature of these peaks for many of the elements studied. The recorded spectra show a general similarity for the elements Sc → Zn but interesting differences emerge. At the beginning of the transition period the L3 based transitions have the relative intensities L3 M2,3, M2,3 > L3 M2,3, M4,5 L3 M4,5, M4,5 whereas towards the end of the series the order L3 M4,5, M4,5 > L3 M2,3, M4,5 L3 M2,3, M2,3 is observed. Pronounced chemical shifts have been observed upon oxidation. The spectra are interpreted in terms of an L -S coupling scheme and the fine structure discussed in terms of effects produced by multiplet splitting.

Surface oxidation of nickel metal as studied by X-Ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to study the oxidation of polycrystalline nickel metal. The results indicate that the oxidation process takes place in three stages; associative adsorption of molecular oxygen, followed by the combination of oxygen atoms with surface nickel atoms and, ultimately, the formation of bulk oxide. At room temperature only the first two stages can be detected. For exposures below 1 L the O 1s photoelectron spectrum is considered to be characteristic of an associatively adsorbed oxygen species, but for exposures above this value evidence for the formation of a monolayer of “NiO” is suggested by the development of an O 1s peak at 529.9 eV. Incorporation of oxygen into the nickel lattice is observed at temperatures >500°K. The activation energy for this place-exchange process was estimated at 1.80±0.06 eV.

The nature of the oxide formed on 20%Cr/25%Ni/Nb steel oxidised at 1123k in co2

Abstract The oxide formed on 20%Cr/25%Ni/Nb stabilised steel at 1123 K in one atmosphere of CO 2 has been characterised using X-ray photoelectron spectroscopy and X-ray diffractometry. A duplex oxide was identified, the outer layer being a Mn(Fe,Cr) 2 O 4 spinel and the inner oxide rhombohedral Cr 2 O 3 . The presence of carbon was monitored through the oxide and its chemical state determined. The outer spinel contained a significant quantity of carbon in the form of a carbide while the inner Cr 2 O 3 layer was essentially carbon-free.

Formation and growth of spinel and Cr2O3 oxides on 20% Cr-25% Ni-Nb stainless steel in CO2 environments

The nature of the first-formed oxide on 20% Cr/25% Ni/Nb stabilized steel during exposure to CO2 at high and low temperatures has been determined by surface analytical techniques. These results together with a consideration of gas/solid interactions show that the oxide produced may be determined by kinetics or thermodynamic factors, and a diagram is presented to show that rhombohedral Cr2O3 or spinel may be the oxide first formed. Under most standard conditions, a mixed spinel oxide is formed initially, and the subsequent growth of a duplex oxide is analyzed in terms of a solid-state reaction in which the spinel oxide is reduced to Cr2O3 at the metal/oxide interface. Diffusion control of growth by either spinel or Cr2O3 is incorporated in new equations describing the kinetics of oxidation, and weight-gain predictions are tested against experimental observations.

Thickness measurements of spinel and chromia layers in stainless steel oxide scales by X-ray diffractometry

An X-ray diffractometric technique has been used to determine the relative thicknesses of the two phases present in the duplex oxide formed on 20% Cr-25%Ni-Nb stabilized steel at 1123 K in CO2.At times < 100 hr an outer spinel layer grows faster than an inner Cr2O3 -rich layer, while after 500 hr the Cr2O3layer is the faster growing. The results indicate that parabolic kinetics do not pertain to the growth of the Cr2O3layer during early oxidation (<500 hr), the rate being controlled by an interface reaction between chromia and the outer spinel.

Oxidation pretreatment to reduce corrosion of 20%Cr-25%Ni-Nb stainless steel. I. Weight gain and oxide thickness measurements

The improvement in corrosion resistance afforded by a low-pressure selective oxidation pretreatment on 20%Cr–25%Ni-Nb steel is assessed in terms of weight gain and oxide thickness measurements. Both can and sheet specimens were oxidized in a simulated CAGR CO2 environment at 823, 923, and 1073 K, and gravimetric gross weight-gain measurements were supplemented by spinel and Cr2O3 oxide thickness measurements determined by X-ray diffractometry (XRD). The increased protection provided by the pretreatment resulted in a reduction in gross weight gain of 3–4 times at 823 K, two and three times at 923 K, and a somewhat smaller improvement at higher temperatures. The improvement stemmed from the high proportion of Cr2O3 selectively formed in the preoxide layer itself. Thermally induced lattice strains in the oxide scale have been assessed from measurements of lattice expansion by XRD.

Surface studies of the interaction of cesium hydroxide vapor with 304 stainless steel

The reaction of cesium hydroxide vapor with 304 stainless steel has been studied over the temperature range 550–950°C in an argon atmosphere containing 3% water vapor and 4% hydrogen. A wide range of surface-analytical techniques has been used to identify the corrosion products, including scanning electron microscopy with energy-dispersive analysis of X-rays, X-ray photoelectron spectroscopy, Auger electron spectroscopy, secondary-ion mass spectrometry, and Raman spectroscopy. A nickel-sputter, ion-plating process was also used to separate the surface oxide layer from the underlying material, thus enabling the exposed interfaces to be examined. At 550°C, the cesium is physisorbed onto the iron hydroxide layer at the surface of the metal. At higher temperatures, most of the cesium is found as isolated cations within the inner chromia lattice, with no evidence for the existence of stable cesium-based compounds.

X-ray photoelectron/Auger electron spectroscopic study of the initial oxidation of chromium metal

Previous work on the oxidation of chromium is briefly reviewed and the theoretical aspects relating to the determination of surface concentrations of metal oxide films by X-ray photoelectron spectroscopy is discussed. An ultra-high vaccum, high resolution X-ray photoelectron/Auger electron spectrometer was used to observe the initial stages of oxidation of clean crystalline chromium following accurate redetermination of the binding energies of chromium core electrons and the work function of the clean metal. High resolution LMM Auger spectra have also been obtained from the clean metal for the first time and show fine structure not previously observed. Using both X-ray photoelectron spectroscopy and Auger electron spectroscopy the rate of formation of oxide films on the surface of the metal in low pressures of pure oxygen and under oxide ion bombardment has been investigated and limiting thicknesses of the films determined.

Oxidation behavior of 20% Cr/25% Ni/Nb stabilized stainless steel in CO2 environments

The oxidation of 20wt.%Cr/25wt.%Ni/Nb steel in 50 ton CO2 at 1073 K has been studied “in situ” using X-ray photoelectron spectroscopy to determine the chemical composition of the oxide initially formed. The surface composition of the first formed oxide is shown to be iron rich, containing quantities of chromium and manganese, whilst analysis of the bulk oxide indicates that the majority of the oxide scale is a spinel of type MnCr2−xFexO4. The formation of a chromia layer, which has been suggested to form first on these steels, was not observed. An examination of the oxide morphology using scanning electron microscopy revealed the presence of particle mounds varying in size from <0.5 μm to ∼3 μm in diameter and embedded in the surface oxide. Other techniques, including scanning Auger microscopy and energy dispersive X-ray analysis, have been employed to determine the composition of these particles, and suggestions for their origin have been offered.